Reintroduce the whitelist for websites linking to flash download page

[Slava]

Fixes the #13500 regression, does not add a regression test. It is not entirely clear to me if a regression test with a google's result page of "flash test" would be very useful but let me know what you think.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

Manually tested the popup behavior on google and a test page

To test:

• install and enable flash, restart the browser
• search "flash test" in google and click the link to the flash download page on adobe website (likely first search result)
• observe that there is no pop-up prompt to allow flash on the google page (the adobe page will have the prompt because it runs an ad with flash)
• go to the test page: https://github.com/Slava/resume/issues/1
• click the same link to the download page
• observe that the browser didn't change the page, but instead prompted to enable flash on that page

 ## Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[diracdeltas]

[minor] please use curly braces around the function body and no semicolon

this would normally be caught by npm run lint but the linter excludes content scripts :(

[diracdeltas]

master / 0.24.x: 22c17a4

[bsclifton]

Uplifted to 0.23.x with 5ab5980